The invention relates to a high-pressure discharge lamp provided with a lamp vessel sealed in a vacuum-tight manner, electrodes arranged in the lamp vessel, current supply conductors extending from the electrodes through the wall of the lamp vessel to the exterior, an ionizable filling in the lamp vessel comprising a rare gas constituent and an evaporable constituent, and a carbon coating on the lamp vessel, which laterally surrounds at least one of the current supply conductors.
Such a lamp is known from GB 615940.
In high-pressure discharge lamps, the wall of the lamp vessel during operation has a comparatively low temperature at the area at which said lamp vessel laterally surrounds a current supply conductor. The evaporable constituent of the ionizable filling can accumulate at this area and can thus be withdrawn from the discharge arc. This influences the spectrum of the radiation emitted by the lamp.
According to the aforementioned British Patent Specification, the lamp vessel therefore has a non-reflecting coating of carbon powder applied by means of a suitable binder. Other powders used for this purpose are thorium oxide and black metal powder, for example tungsten powder. The increase in temperature of the coated wall portion is obtained according to this Patent Specification in that the coating assumes a comparatively high temperature due to absorption of radiation generated by the lamp, although the coating itself supplies energy to the environment by radiation.
A disadvantage of the known coating and of other usual powder coatings, such as ZrO.sub.2 powder coatings, it that it is difficult to manufacture the coating in a reproducible manner, that suspension agents are used which must be expelled later on and that it may be necessary to manufacture the coating in two cycles of immersing or smearing and baking. Due to the powder particles in the coating, the coating has a rough surface and is dull black when carbon is used. There is a risk that the coating keeps volatile constituents absorbed or adsorbed, which are eventually released or desorbed during operation of the lamp. When the lamp burns in an evacuated outer envelope, the vacuum is reduced due to this desorption.
Further, the adhesion of such a coating to the lamp vessel is poor, as a result of which the coating is liable to be damaged.
The aforementioned British Patent Specification further indicates that it is known to provide a lamp vessel locally with a reflecting gold or platinum layer and that the use of these metals is expensive.